1. Field of the Invention
The present invention relates to a D.C. cast ingots of aluminum alloy available for rolling operation so that Al-Fe intermetallic compound is crystallized and moreover relates to a method of manufacturing said D.C. cast ingots.
2. Description of the Prior Art
Generally direct chill (D.C.) cast ingots of aluminum alloy containing Fe undergo the following processes. First their surface area is chipped off by a predetermined thickness (usually about 5-7 mm) so that a so-called coarse cell zone developed on the surface area of the respective D.C. cast ingots is removed. This coarse cell zone represents the zone where dendrite arm spacing in the D.C. cast ingots is large. Provided that it is rolled without any removal of the coarse cell zone, the result is that degraded rolled sheet or plate will be produced. Therefore the coarse cell zone should be removed prior to rolling operation. Then the cast ingots with the surface area thereof removed is subjected to rolling operation. Next the rolled product in the form of sheet or plate undergoes anodizing treatment. As far as the conventional D.C. cast ingots of aluminum alloy containing Fe is concerned, it has been sometimes recognized that the anodized product has a band-shaped pattern of a different colour on its outer surface.
It is well know that the aforesaid band-shaped pattern caused by anodizing is attributable to a so-called fir-tree structure which is occurence within the D.C. cast ingots.
In fact the term "fir-tree structure" designates a particular fire-tree shaped macro-structure which is developed in a D.C. cast ingots. Specifically, it is often recognized with the continuous D.C. cast ingot that when they are cut in the casting direction and then their exposed cut surface is subjected to anodizing, a dark or dark grey fir-tree shaped pattern developes on the cut surface as shown in FIGURE. This macro-structure having a fir-tree shaped pattern is referred to as fir-tree structure.
When the D.C. cast ingots with the fir-tree structure contained therein is rolled with its surface area chipped off in the above described manner, the sheet or plate produced by the rolling has a pattern comprising a fir-tree structure region (A) and a non-fir-tree structure region (B) which are alternately located. Next, when the rolled sheet or plate is subjected to anodizing treatment, its surface appears dark or dark grey over the region (A), while it appears light grey over the region (B). As a result the surface of the rolled sheet or plate shows the band-shaped pattern as described above.
Once any pattern developes over a surface of rolled sheet or plate which has been subjected to anodizing treatment, the sheet or plate is unavoidably rejected as a worthless product because of its unattractive appearance, resulting in substantially reduced productivity. Rejected sheet or plates are remelted to recover aluminum material in the form of D.C. cast ingot, but a part of remelted material is oxidized during a process of remelting, degassing and other treatment. It should be emphasized that aluminum alloy material usually has a remarkably high level of material loss due to oxidation.
In order to prevent any formation of band-shaped pattern on the rolled sheet or plate it is advisable that all D.C. cast ingots are sliced off in the transverse direction prior to rolling operation to check as to how the fir-tree structure developes in the respective D.C. cast ingots. This is intended to determine a thickness to be chipped off over the upper and lower surfaces of the respective D.C. cast ingots and moreover to ensure that either of a fir-tree structure area and a non-fir-tree structure area appears across the cut surface thereof. A problem is, however, that the above described procedure results in a substantially reduced productive efficiency.
In view of the background as described above, anxious requirements have been raised for remedial measures against development of the fir-tree strucutre in the D.C. cast ingots which has been considered as a fundamental cause of formation of bandshaped pattern.
D. Altenpol had been so stated on his previous report (Zeit. fur Metallkde., 46 (1956), 536) that above mentioned "fir-tree structure" (he so called "Tannenbaumastes") is attributable to a segregation phenomenon on a process of solidification in the D.C. casting. However, later workers have been presented that said structure are due to a form of crystallized Al-Fe intermetallic compounds in the D.C. cast ingots. In practice, however, no satisfactory clarification has been reached on the phenomenon of development of the fir-tree structure. At present an acceptable presumption is that development of the fir-tree structure is attributed to the fact that Al-Fe intermetallic compounds having different characters are crystallized in different regions in a cast ingot. Specifically, Al.sub.6 fe is crystallized in the fir-tree structure region (A), while Al.sub.3 Fe and AlmFe is crystallized in the non-fir-tree structure region (B), wherein the preceding suffix m denotes the number which is neither 3 nor 6. It seems to that Al.sub.6 Fe crystals are not dissolved in H.sub.2 SO.sub.4 aqueous solution treatment so that the crystals are retained within the anodic oxide film and the existence of Al.sub.6 Fe crystals causes the film to appear dark or dark grey. In the meanwhile, it is presumed that Al.sub.3 Fe and AlmFe crystals are completely dissolved in H.sub.2 SO.sub.4 aqueous solution without any residue in the film of anodic oxide film, which causes said anodic oxide film to appear light grey.
The aforesaid crystallization of Al-Fe intermetallic compound is dependent on the rate of solidification of molten metal (i.e. cooling rate). It has been experimentally confirmed that Al.sub.3 Fe is crystallized when molten metal is solidified slowly, and Al.sub.6 Fe is crystallized when it is solidified quickly, and AlmFe is crystallized when it is solidified more quickly.
This suggests that it is possible to reduce an area of region where Al.sub.6 Fe is crystallized, that is, the region of fir-tree structure by way of the step that molten metal is solidified slowly. In other words, it is possible to increase the distance l showed in FIG. 1 from the surface of the D.C. cast ingot to the boundary of the fir-tree structure in a same manner. A problem is, however, that solidification slowly causes substantially reduced productive efficiency.
Another approach to eliminating of the fir-tree structure is to continuously heat treat cast ingots. Al.sub.6 Fe crystals in the region (A) of the fir-tree structure has a metastable phase which is thermally instable. By heating it for a period longer than 4 hours at a temperature of 620.degree. C. it is transformed into Al.sub.3 Fe which has a stable phase. It has been recognized as drawbacks with this process that production efficiency is reduced and manufacturing installation is relatively expensive. As a result it's difficult for us to employ this process for actual production at present.